It depends on what you intend to do with the lambs if all are to be killed for meat then ovivac p or heptavac p are fine , if you intend to keep some for future breeding then heptavac p should be your choice as it covers lamb dysentery which can be a problem in some flocks at birth . Both products are basically the same ovivac p covers four diseases + pneumonia and heptavac p seven diseases + pneumonia , so if hep is cheaper then it would seem the way to go . Yes you are correct with your timings.